Single detector differential particulate mass monitor with intrinsic correction for volatilization losses

ABSTRACT

The mass of particulate matter in a particle laden gas stream is measured using a single mass detector. The particle laden gas stream and a substantially identical but particle-free gas stream alternately engage the mass detector during successive measurement time periods. A difference between a reading provided by the mass detector for a current measurement time period and a reading provided by the mass detector for a consecutive measurement time period is determined. This difference intrinsically corrects for volatilization losses occurring during the current measurement time period. A measure of the mass or concentration of particulate matter in the particulate laden gas stream is determined from this difference.

This application claims the benefit under 35 U.S.C. 119(e) of U.S.provisional application 60/133,320 filed May 10, 1999.

TECHNICAL FIELD

This invention relates, in general, to the measurement of particulatematter suspended in a fluid medium and, more specifically, tomeasurement of the mass and/or concentration of particulate mattersuspended in ambient air or in other gaseous environments, e.g. indiesel exhaust, or in mines, smoke stacks, industrial facilities, etc.Particulate matter is the general term which refers to condensed solid,semi-solid, or liquid material produced as a result of natural orman-made processes, and which due to small size, is capable of beingsuspended in the air or other fluid medium.

BACKGROUND ART

The measurement of particulate matter in ambient air is important for avariety of reasons, the most important of which is related to healtheffects. Suspended particulate matter is known to produce a variety ofdeleterious health effects when inhaled. As a result, regulatoryagencies around the world require monitoring of the levels ofparticulate matter. The levels are measured in terms of concentration,i.e. micrograms of particulate matter per cubic meter of air. Referencetechniques for this measurement are presently defined in terms of a massmeasurement utilizing a filter medium to capture the particulate matterand the total volume of air which has been filtered by the medium over agiven period of time. There are various means available to unambiguouslydetermine the flow rate through the filter over time (and hence thevolume of air sampled), but surprisingly the mass measurement is notstraightforward due to the complex nature of ambient particulate matterwhich results in unstable mass deposition on the filter.

This problem involving the measurement of particulate matter in ambientair is well-known. The uncertainty arises since the particulate massused as a basis for mass concentration computations is defined as themass captured on the filter media which is not necessarily the mass ofthe particles as they exist in the ambient air. Unlike measurements ofmajor criteria gaseous pollutants, what is defined as particulate mattercan change its mass as a result of loss or gain of volatile substancesassociated with the particulate matter and filter media. While gaseouspollutants exist as definable molecular species (SO₂, O₃, CO, etc.),particulate matter can be a combination of different substances withdifferent volatilization rates, reactive, desorptive, absorptive, andadsorptive properties. In addition, the mass of particulate matter onthe filter can be affected by the filter material itself, theparticulate matter already collected on the filter, the face velocitythrough and pressure drop across the filter, as well as by the humidity,temperature and composition of the gas stream passing through thecollection medium.

Both direct and indirect measurement techniques have been employed in aneffort to quantify particulate matter mass. Each method which has beendeveloped to date, however, has limitations in obtaining a measurementof the actual mass of particulate matter as it exists in its suspendedform. Direct mass measurements as represented by weighing materialcaptured on a substrate such as a filter are susceptible to instrumenteffects due, for example, to temperature or pressure changes, and tovolatile component losses which are not easily quantifiable. Indirectmethods such as light scattering measurements on the other hand areinherently inaccurate as there is no physical connection between otherproperties of particles and particle mass.

To compensate for instrument effects in direct mass measurements, adifferential particulate mass measurement microbalance employing a pairof oscillating quartz crystal detectors has previously been proposed. Inthis earlier approach, a particle laden gas stream impacts upon thefirst detector and a particle free gas stream impacts the seconddetector. The second mass detector is used as a reference to cancel outdetector instrument effects from a mass reading provided by the firstdetector. U.S. Pat. No. 5,571,945 discloses a similar measurementapproach employing a pressure sensor to measure a pressure differentialbetween a pair of particulate matter collectors; U.S. Pat. No. 5,349,844discloses a similar approach for use with a filter that is caused tooscillate in a direction substantially perpendicular to a plane of thefilter. However, volatilization losses are not accounted for in theseearlier systems.

As a result of the above described difficulties, the current referencemethod in the United States is a method dependent technique which doesnot necessarily represent an accurate measure of particulate mass as itactually exists in its undisturbed state in the air. The referencemethod consists of filter equilibration under a defined range oftemperature and humidity conditions, a pre-collection weighing of thefilter, the installation of the filter in a manual sampler, the samplingof ambient air (for a 24-hour period), the removal of the filter fromthe sampling device, a post-collection conditioning under the sameequilibration conditions as before, and finally a post-collectionweighing. This methodology is intended to provide a consistent set ofmeasurements between identical samples.

However, for the reasons stated above, results based on this method donot represent measurements to which an accuracy can be assigned, evenloosely, i.e. to what accuracy is the particulate mass as it exists inthe atmosphere measured by the mass determined from the filter? This isa serious problem, and one has to accept the fact that thesemeasurements are only an indication of particulate levels. As a result,the current reference method represents simply a standardized procedure,and not a scientifically-based measurement standard for airborneparticulate matter.

Volatile components are a confounding influence on these measurements.While the filter resides in the sampling hardware, important factorsthat influence the reactions taking place on the filter substrate, suchas temperature and humidity, vary in an ill-defined manner. Duringsampling, the mass on and of the filter can increase dramatically duringperiods of decreasing temperature and increasing relative humidity(nighttime), and may experience substantial loss of semi-volatilematerials when the temperature increases and humidity decreases(daytime). These same type of effects can be associated with air masschanges, and other meteorological events. Further, the collection filtermay be exposed to widely varying hot or cold temperatures once samplingis complete and before it is removed from the sampler as well as duringtransportation to a laboratory for conditioning and weighing.

Not only does the mass of collected particulate matter and the filterchange depending upon the conditions to which they are exposed, but theair stream through the filter creates a pressure differential across thefilter which tends to strip off volatile components of the particulatematter. In effect, the interaction of the particles with the filtertends to modify the nature of the particulate matter as soon as it iscollected, thereby affecting the accuracy of the desired measurement ofthe particulate matter as it is suspended in ambient air. As healthconcerns heighten, and measurement instrumentation becomes moresensitive, there is a trend towards measurement of even finerparticulate matter, e.g. particles of 2.5 microns or less. With smallerparticles, the impact of volatilization losses upon the mass measurementreadings becomes even more pronounced.

A compelling need thus exists for a measurement instrument that canaccurately measure the mass or concentration of particulate mattersuspended in ambient air or other gaseous environments.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus which overcomesthe problems described above and provides a collection-based direct massmeasurement that allows the accurate quantification of the mass ofambient air or other gas borne particulate matter including volatilecomponents thereof. The measurement approach of the present inventionnot only cancels out detector instrument effects but also intrinsicallycorrects for volatilization losses. For purposes of this disclosure, theterm “volatilization losses” is used broadly to include vaporization,absorption, adsorption, desorption, reactive and other effects whichinfluence (positively or negatively) the mass of collected particulatematter. Some of the effects are generally known as collector (e.g.filter) artifacts.

In accordance with the principles of the present invention, apparatusfor measuring the mass of particulate matter in a particle laden gasstream includes a mass detector, and first means for providing aparticle free gas stream otherwise substantially identical to theparticle laden gas stream. Switching means causes said particle ladengas stream and said particle free gas stream to alternately engage saidmass detector during successive measurement time periods. A differencebetween a reading provided by the mass detector for a currentmeasurement time period and a reading provided by the mass detector fora consecutive measurement time period is computed. This differenceintrinsically corrects for volatilization losses occurring during thecurrent measurement time period. A measure of the mass or concentrationof particulate matter in the particle laden gas stream is determinedfrom this difference.

Advantageously, the first means for providing a particle free gas streamcomprises particle removal means for removing substantially allparticulate matter from said particle laden gas stream. Optimally, saidparticle removal means removes the particulate matter from the particleladen gas stream without appreciably affecting gas stream temperature,pressure and flow rate. Such particle removal is preferably accomplishedusing an electrostatic precipitator. The precipitator preferablyoperates with a positive corona and low current.

The mass detector may comprise an oscillating element microbalance. Thedetector may comprise a hollow element oscillating in a clamped-freemode, with a filter supported at a free end of the element. The filterserves to collect particulate matter from the particle laden gas streamwhen this stream engages the detector. Fluid control means canadvantageously maintain a substantially constant gas stream flow at thefilter of the detector during each measurement period. In theoscillating element microbalance embodiment, mass readings provided bythe mass detector are advantageously based upon detected change offrequency of oscillation of the oscillating element with respect totime.

In another aspect of the invention, the switching means of the massmeasuring apparatus causes: (a) the particle laden gas stream to engagethe mass detector during each of odd numbered ones of the successivemeasurement time periods, and (b) said particle free gas stream toengage the mass detector during each of even numbered ones of thesuccessive measurement time periods. When the detector is engaged by theparticle laden gas stream, it measures mass gain; when the detector isengaged by the particle free gas stream, it measures mass lost due tovolatilization of volatile components of the particulate matter. Themeasured mass lost is added to the measured mass gain to determine themeasure of the mass of the particulate matter. Each successivemeasurement time period lasts for a short time, preferably fifteenminutes or less; about a minute or less being presently considered asmost preferred.

In another aspect of the invention, the readings provided by the massdetector each comprise a mass rate reading, which limits accumulation ofany calibration errors in the mass measurement.

In yet another aspect of the present invention, corrected massconcentration is computed from a corrected mass rate which combines massrate readings from the mass detector for two successive measurementperiods.

The present invention also presents a significant improvement toexisting differential particle mass measurement systems. In suchsystems, a particle laden gas stream engages a first mass detector and aparticle free gas stream engages a second mass detector. The second massdetector is used as a reference to cancel out detector instrumenteffects from a reading provided by the first mass detector. According tothe present invention, a differential particle mass measurement systemis improved by inclusion of switching means for causing the particleladen gas stream and the particle free gas stream to alternately engagea single mass detector, during successive measurement time periods. Inthis fashion, correction is intrinsically provided for volatilizationlosses occurring during the successive measurement time periods,calibration or matching of multiple detectors is avoided, and thecomplexity and cost of the measurement device is reduced.

In a further aspect of the present invention, apparatus for measuringthe mass of particulate matter, including volatile components thereof,in a particle laden gas stream is provided. This apparatus includes amass detector, means for directing the stream to continually engage themass detector, and particle removal means for removing substantially allparticulate matter from the stream when the particle removal means isactivated. Control means activates the particle removal means foralternate successive measurement time periods. A difference isdetermined between a first reading provided by the mass detector and asecond reading provided by the mass detector for successive measurementtime periods. This difference intrinsically corrects for volatilizationlosses occurring during the measurement time periods. A measure of themass or concentration of particulate matter in the particle laden gasstream is determined from this difference.

Pursuant to a still further aspect of the present invention, adifferential particle mass measurement method is improved. In the knownmethod, a particle laden gas stream engages a first mass detector. Thefirst mass detector collects a current particle sample from the gasstream during a current measurement time period and measures mass gaindue thereto. A second mass detector is used as a reference to cancel outdetector instrument effects. The present invention improves upon thismethod by using a single mass detector to not only cancel out detectorinstrument effects but to also effectively measure a change in particleproperty occurring during said current measurement time period. Thischange in particle property usually comprises a loss of mass due tovolatilization of collected volatile particles. The mass lost due tovolatilization as measured by the mass detector is added to the massgain measured by the mass detector to yield a corrected particle massmeasurement for the current measurement time period. The measured lossof mass occurs during a consecutive measurement time period (i.e.,during a period just before or after the current measurement timeperiod) in an earlier collected particle sample; this earlier collectedparticle sample having been collected by the mass detector in apreceding measurement time period. Preferably, the current measurementtime period and the consecutive measurement time period are of suchshort duration as to ensure substantially identical volatilizationduring said consecutive measurement time period of the earlier collectedsample and the current particle sample.

The present invention provides numerous significant benefits andadvantages. Foremost among these is its intrinsic correction forvolatilization losses occurring during measurement time periods. Usingshort measurement time periods ensures that the particulate massmeasurement includes an accurate representation of the volatile massassociated with the collected particulate under any selected temperatureincluding varying ambient temperature conditions. Since the massdetector sees substantially identical collector (e.g. filter) andinstrument artifacts during a pair of successive measurement timeperiods, compensation for instrument effects is effective and completewhen the readings from one measurement time period are subtracted fromthose of the other. The preferred use of an electrostatic precipitatorfor particle removal prevents any pressure disturbance from occurring.The electrostatic precipitator also facilitates switching of theparticle content of the gas stream on and off electrically andinstantaneously with no mechanical motion being necessary. The switchingof the gas stream also effectively expands collector (e.g. filter) lifeby a factor of 2 compared to a continuous particle collector system.Further, if two differential measurement instruments are runside-by-side in accordance with the principles of the present invention,one at ambient temperature and the other at a significantly elevatedtemperature, then a division between volatile and non-volatilecomponents of the ambient particulate matter can be obtained. The use ofa single detector provides additional cost and reliability benefits.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features and advantages of the presentinvention will be more readily understood from the following detaileddescription of preferred embodiments when read in conjunction with theaccompanying drawing figures in which:

FIG. 1 is a simplified schematic illustration of one embodiment of adifferential particulate mass measuring apparatus of the presentinvention;

FIG. 2 is a graphical depiction useful in understanding how correctedmass concentration can be derived from mass rate readings of thedetector of the differential particulate mass measuring apparatus of thepresent invention; and

FIG. 3 illustrates an alternate embodiment of the apparatus of thepresent invention.

DETAILED DESCRIPTION

The goal of the present invention is to accurately measure the mass ofparticulate matter suspended in ambient air (or other gaseousenvironment) including volatile components thereof. This is accomplishedby causing a particle laden gas stream and a substantially identical butparticle free gas stream to alternately engage a mass detector duringsuccessive measurement time periods. With suitably short measurementtime periods, taking a difference between successive readings providedby the mass detector, in effect adds to the measured mass gain of thecollected particulate matter, the mass lost due to volatilization duringthe measurement time interval thereby providing an accurate measurementof total particulate mass in the gas stream.

A first embodiment of the particulate mass measurement instrument of thepresent invention is schematically illustrated in FIG. 1. The instrument10 includes an inlet 12 in pneumatic communication with a selectivelyactivatable particle remover 14 and a mass detector 16. A particle ladengas stream 18 enters the inlet 12, passes through the selectivelyactivatable particle remover 14 and continually engages the massdetector 16. The term “engages” is used herein to broadly connote theinteraction of a gas stream with a mass detector. Such interaction maytake many different forms depending upon the nature of the mass detectoremployed.

Inlet 12 would normally include a separator for pre-separating out ofthe particle laden gas stream, particles larger than a predetermined“cut-off” size. Inlets with PM10 and/or PM2.5 separators, for example,are well known and commercially available.

The particle laden gas stream exiting inlet 12 is then channeled to andthrough selectively activatable particle remover 14. When activated,particle remover 14 removes substantially all particulate matter fromthe gas stream, without appreciably affecting gas stream temperature,pressure and flow rate. Such particle removal can be advantageouslyimplemented using an electrostatic precipitator of the same general typeas is commonly used in air cleaning equipment. In order to reduce ozoneproduction, an electrostatic precipitator operating with a positivecorona and very low current, e.g. on the order of tens to hundredsnanoamps, is preferred. The current should be sufficient to cause theprecipitator to remove substantially all particulate matter from the gasstream.

Particle remover 14 is selectively activated during alternate successivemeasurement time periods to remove particulate matter from gas stream 18during the alternate successive measurement time periods.

For example, for a first measurement time period, particle remover 14may be turned on. During the next measurement time period, particleremover 14 is turned off. This alternating activation pattern continuesfor successive measurement time periods. Advantageously, each timeperiod is of relatively short duration, preferably on the order offifteen minutes or less, more preferably on the order of five minutes orless, and most preferably on the order of one minute or less.

The gas stream 18 emerging from the particle remover 14 continuallyengages mass detector 16 throughout the successive measurement timeperiods.

Although other direct mass or indirect mass detectors such as quartzcrystal microbalances, beta absorption monitors, pressure drop monitors,etc. may also be used, the preferred implementation of mass detector 16is a tapered hollow element oscillating microbalance. The latterinstrument is preferred because of its high mass sensitivity, real-timecapability, direct inertia-based mass measurement, and high collectionefficiency utilizing filtration.

High mass sensitivity is important since the amount of mass which needsto be measured is in the microgram and sub-microgram range. Real-timemeasurement is important since mass volatilization can occur in shorttime frames. Direct mass measurement is desirable to avoid ambiguity inwhat is measured and also allows for a mass standard traceablecalibration. Filtration ensures high collection efficiency. A suitabletapered element oscillating microbalance is described in commonlyassigned U.S. Pat. No. 4,391,338, with further background informationprovided in U.S. Pat. No. 3,926,271. The teachings of these two patents,in their entirety, are incorporated herein by reference.

In the preferred microbalance, a tapered hollow element is made tooscillate in a clamped-free mode. A filter is mounted at a free end ofthe element and serves to collect particulate matter from the particleladen gas stream. When this stream passes through the filter and thenthrough the hollow element, the frequency of oscillation of the hollowelement varies with mass loading of the filter and is readilyconvertible to a mass reading. For purposes of the present invention,changes in frequency of oscillation of the oscillating element withrespect to each measurement time period is advantageously converted to amass rate (i.e. change in mass with respect to the measured timeinterval) to determine a corrected mass concentration, as more fullydescribed hereinafter.

Although a tapered hollow element oscillating microbalance is preferred,oscillating elements of other forms or configurations, e.g. non-tapered,tuning fork or U-shaped, or one operating in another mode, e.g.clamped-clamped mode, or a collector made to oscillate in a directiongenerally perpendicular to a plane of the collector, or an impactionplate or other particle collector, may also be employed as the massdetector.

In operation, with the particle remover 14 off, the mass detector 16will measure an effective mass, M_(Aeff), due to frequency changesresulting from mass changes and instrument effects, over a time period,Δt:

M _(Aeff) =M _(p) +M _(pv) +αM _(G) +βΔT+γΔP  (1)

where:

M_(p)=non-volatile component of particulate mass

M_(p)=volatile component of particulate mass

αM_(G)=gaseous mass gain or loss due to filter adsorption/desorption andother filter artifacts, and deposited material volatilization

βΔT=effective mass equivalent of frequency change due to temperaturechange, ΔT, during time interval, Δt

γΔP=effective mass equivalent of frequency change due to pressurechange, 66 P, during time interval, Δt.

During a consecutive time interval, Δt, particle remover 14 is on, i.e.M_(p) and M_(pv) are removed from the gas stream and are not measured onmass detector 16, and:

M _(Beff) =αM _(G) +βΔT+γΔP  (2)

Therefore:

M _(Aeff) −M _(Beff) =M _(p) +M _(pv) +αM _(G) +βΔT+γΔP−(αM _(G)+βΔT+γΔP)=M _(p) +M _(pv)  (3)

These measurement time intervals Δt are kept relatively short incomparison to the rate of change in particulate concentration and inother mass measurement affecting parameters, e.g. temperature andpressure, so that vaporization effects, the adsorption/desorptioneffects of gases or vapors in the gas stream, other filter artifacts,and temperature and pressure change effects remain comparable during the2 successive measurement time periods, allowing an accurate subtraction.

The mass detector 16 when the particle remover 14 is not activatedduring a particular measurement time period, provides a mass reading,i.e. mass gain, representative of the mass of particulate mattercollected during that time period. An effective measure of the mass lostdue to volatilization during that same time period is provided by themass detector during a consecutive measurement time period during whichthe mass detector 16 is engaged by the particle free (particle removeractivated) gas stream. Subtracting the two mass readings, effectivelyadds the lost mass to the measured mass gain to provide a corrected andaccurate measure of the particulate matter mass.

In practice, the measurements can be based on mass rates utilizing theswitching time as a convenient time basis. By subtracting mass rates, asopposed to total mass measurements, any slight calibration error in themeasurement system will produce only a comparable final error in theresults. If mass were strictly used, this error could accumulate tounacceptable levels as monitoring time continues.

FIG. 2 depicts an example of how mass might vary over four successivemeasurement time periods as measured by the detector 16 of theinstrument of the present invention. A corrected mass concentration canbe calculated in accordance with the following relationships:

(−1)^(n+1)(Δm _(B) /Δt)_(n)+(−1)^(n)(Δm _(A) /Δt)_(n)=(Δm/Δt)_(n)  (4)

and $\begin{matrix}{({MC})_{n} = {{\left( {\Delta \quad {m/\Delta}\quad t} \right)_{n}\left( {1/F} \right)} = \frac{\Delta \quad m}{\Delta \quad V_{n}}}} & (5)\end{matrix}$

where:

Δt represents a time interval of a measurement time period,

n represents a measurement time period index, for even n's, the particlefree gas stream engages the mass detector, and for odd n's, the particleladen gas stream engages the mass detector,

Δm_(A)/Δt represents mass rate measured by the mass detector during anodd n measurement time period,

Δm_(B)/Δt represents mass rate measured by the mass detector during aneven n measurement time period,

Δm/Δt represents corrected mass rate,

ΔVn represents the volume of gas sampled during measurement period n,

F represents flow, and

MC represents corrected mass concentration.

In equation (4) above, the indexing to (−1) effectuates a subtraction ofthe readings.

FIG. 3 illustrates an alternate embodiment of the differentialparticulate mass measurement apparatus of the present invention. Inmeasurement instrument 40, a particle laden gas stream 42 is drawn inthrough a particle size selective inlet 44, of conventionalconstruction, and then is split into two streams 46A, and 46C. Particleladen gas stream 46A flows through an optional dryer or dehumidifier 47(e.g. a Perma Pure PD™-Series gas dryer of Nafion® constructionavailable from Perma Pure Inc. of Toms River, N.J.), then through aselectively activatable particle remover (e.g. electrostaticprecipitator) 48, and then engages mass detector (e.g. tapered elementoscillating microbalance) 50. Dryer 47 advantageously serves to reduce,control, or eliminate water vapor in gas stream 46A. Stream 46Crepresents a by-pass flow to allow proper flow rate through inlet 44.Flow controllers 52A and 52C serve to maintain a desired constant flowrate in stream 46A and at mass detector 50. For example, if the flowrate out of inlet 44 is 16.7 l/min, flow controller 52A can maintain aflow rate of 2.0 l/min in stream 46A, while flow controller 52Cmaintains a flow rate of 14.7 l/min in stream 46C. A common vacuum pump54 working in conjunction with the flow controllers establishes thedesired flow. A particle remover switch 56 activates particle remover 48for alternate successive measurement time periods as directed bycontroller 58. Controller 58 which can readily be implemented by amicrocomputer or other known processor, also controls a frequencycounter data analyzer 60 which receives frequency readings from massdetector 50 and, in known fashion, transforms such frequency readingsinto mass readings. From these mass readings, a measure of the massand/or concentration of particulate matter in the particle laden gasstream 42 is determined, as earlier described, by analyzer 60, and thenprovided to output device 62. As will be readily apparent to thoseskilled in this art, switch 56, controller 58, data analyzer 60, outputdevice 62 and the other components of this apparatus can take manydifferent forms.

The operation of instrument 40 is identical to that described earlierfor instrument 10.

Although preferred embodiments have been described and depicted herein,it will be readily apparent to those skilled in the art that variousmodifications, substitutions, additions and the like can be made withoutdeparting from the spirit of the invention. For example, switching ofthe particle free gas stream and the particle laden gas stream withrespect to the mass detector can be accomplished by various alternativeapproaches including mechanically switching, e.g. with a valve, betweena particle laden and a particle free gas stream, or mechanicallyswitching the location of the detector. Rather than an electrostaticprecipitator, a filter, e.g. a low pressure drop electrolet filter orother particle remover can be employed to create the particle free gasstream. Depending upon the intended application and operatingconditions, a denuder to reduce gaseous component(s), temperature and/orhumidity control equipment 64 (such as that taught in copending,commonly assigned U.S. Ser. No. 09/014,252, now U.S. Pat. No. 6,151,953incorporated, in full, herein by reference), and other gas stream and/orparticulate matter conditioning apparatus may also be used with thedifferential particulate mass monitor. The readings provided by the massdetector may comprise frequency, mass, mass rate, mass concentration orother parameter(s).

What is claimed is:
 1. Apparatus for measuring the mass of particulatematter in a particle laden gas stream, comprising: a mass detectorincluding a particulate matter collector; first means upstream of saidmass detector for providing a particle free gas stream otherwisesubstantially identical to said particle laden gas stream; switchingmeans for causing said particle laden gas stream and said particle freegas stream to alternately engage said mass detector during successivemeasurement time periods; and second means for determining a differencebetween a first reading provided by the mass detector for a currentmeasurement time period and a second reading provided by the massdetector for a successive measurement time period, which differenceintrinsically corrects for volatilization losses of collectedparticulate matter from said collector occurring during the currentmeasurement time period, and for determining from said difference ameasure of the mass or concentration of particulate matter in theparticle laden gas stream.
 2. The apparatus of claim 1 wherein saidfirst means comprises particle removal means for removing substantiallyall particulate matter from said particle laden gas stream to providesaid particle free gas stream.
 3. The apparatus of claim 2 wherein saidparticle removal means removes said particulate matter from saidparticle laden gas stream while maintaining gas stream temperature,pressure and flow rate substantially the same.
 4. The apparatus of claim3 wherein said particle removal means comprises an electrostaticprecipitator.
 5. The apparatus of claim 4 wherein said electrostaticprecipitator operates in a low current, positive corona mode.
 6. Theapparatus of claim 1 further comprising a dryer for reducing water vaporin the particle laden gas stream and in the particle free gas stream. 7.The apparatus of claim 1 wherein a reading provided by the mass detectorcomprises a mass rate rating.
 8. The apparatus of claim 1 wherein themass detector comprises an oscillating element microbalance.
 9. Theapparatus of claim 1, wherein the switching means causes: (a) theparticle laden gas stream to engage the mass detector during each of oddnumbered ones of the successive measurement time periods, and (b) saidparticle free gas stream to engage the mass detector during each of evennumbered ones of the successive measurement time periods; and wherein,when the detector is engaged by the particle laden gas stream, thedetector measures mass gain, and, when the detector is engaged by theparticle free gas stream, the detector measures mass loss; and whereinthe second means effectively adds the measured mass loss to the measuredmass gain to determine the measure of the mass or concentration of theparticulate matter in the particle laden gas stream.
 10. The apparatusof claim 9 wherein the mass lost represents mass of volatile componentof said particulate matter.
 11. The apparatus of claim 10 wherein eachof said successive measurement time periods lasts for no more than aboutfifteen minutes.
 12. The apparatus of claim 10 wherein each of saidsuccessive measurement time periods lasts for no more than about aminute.
 13. The apparatus of claim 1 wherein said measure of the mass ofparticulate matter in the particle laden gas stream comprises acorrected mass concentration determined in accordance with the followingrelationships: (−1)^(n+1)(Δm _(B) /Δt)_(n)+(−1)^(n)(Δm _(A)/Δt)_(n)=(Δm/Δt)_(n) and$({MC})_{n} = {{\left( {\Delta \quad {m/\Delta}\quad t} \right)_{n}\left( {1/F} \right)} = \frac{\Delta \quad m}{\Delta \quad V_{n}}}$

 where: Δt represents a time interval of a measurement time period, nrepresents a measurement time period index, for even n's, the particlefree gas stream engages the mass detector, and for odd n's, the particleladen gas stream engages the mass detector, Δm_(A)/Δt represents massrate measured by the mass detector during an odd n measurement timeperiod, Δm_(B)/Δt represents mass rate measured by the mass detectorduring an even n measurement time period, Δm/Δt represents correctedmass rate, ΔVn represents the volume of gas sampled during measurementperiod n, F represents flow, and MC represents corrected massconcentration.
 14. The apparatus of claim 1 wherein said first meanscomprises a filter.
 15. The apparatus of claim 14 further including atleast one of a temperature controller and a humidity controller.
 16. Theapparatus of claim 14 including both a temperature controller and ahumidity controller.
 17. Apparatus for measuring the mass of particulatematter, including volatile components thereof, in a particle laden gasstream, comprising: a mass detector including a particulate mattercollector; means for directing said stream to continually engage saidmass detector; particle removal means upstream of said mass detector forremoving substantially all particulate matter from said stream when saidparticle removal means is activated; and control means for activatingsaid particle removal means for alternate successive measurement timeperiods; and second means for determining a difference between a firstreading provided by the mass detector for a current measurement timeperiod and a second reading provided by the mass detector for asuccessive measurement time period, which difference intrinsicallycorrects for volatilization losses of collected particulate matter fromsaid collector occurring during the current measurement time period, andfor determining from said difference a measure of the mass orconcentration of particulate matter in the particle laden gas stream.18. The apparatus of claim 17 wherein said particle removal meanscomprises an electrostatic precipitator.